The invention relates to a method of producing copper-chromium fusion alloys, having a chromium content of at least 25% and at most 60% by mass with a homogeneous macrostructure without lattice defects and without shrink holes or pipes, for use as contact material for vacuum power switches with breaking currents above 10 kA. A starting material for the fusion process is prepared from the alloy components copper and chromium, and then this starting material is melted to form the fusion alloy.
Contact materials for vacuum power switches with breaking currents above 10 kA must, besides the reliable current cutoff, fulfill additional requirements. The contact material must manage high permanent currents of several thousands amperes without an undue temperature increase in the switching tube caused by the resulting dissipated heat. In the interests of an easy and silent drive, there must be little tendency to weld. The contact burnoff should be low enough so that at least 100 short-circuit breaks and 10,000 breaks at nominal current are ensured, with the contact exhibiting a smooth-surface burnoff behavior, for dielectric reasons. The texture and composition of the contact material should be such that the breaking current distribution curve is as narrow as possible and the most frequent value is no higher than 4 A. Lastly it must be assured that as the contact material melts and evaporates, the arc will not release gas components which would lead to a critical pressure increase in the switching chamber of above 10.sup.-4 mbar.
A powder-metallurgically produced sintering/infiltration material which contains chromium and copper as base materials is commonly used as as contact material for vacuum power switches. It is produced from especially pure starting materials under shield gas and/or under vacuum. However, even if it is produced under conditions of high vacuum, perturbation zones and defects occur in the structure which are attributable to the reactivity of the chromium. In the infiltration (impregnation) process, these defects lead to faulty wetting on individual grain surfaces of the skeleton. Such deviations from the ideal structure may affect the burning and movement of the arc on the contact surfaces and may impair the current breaking capacity and voltage strength.
In general these disturbing structural defects are attributable to the presence of stable residual oxides on the outer surfaces of the metal powders used such as the chromium.
Another defect, which also may cause failure of the switching tube, is based on impurities within the chromium grains of the starting material or of non-metallic admixtures in the chromium powder. In electrolytic chromium powders there have occasionally been found in the interior of the grains electrolyte residues, and in the case of aluminothermally produced chromium powders, inclusions or impurities of the powder in the form of Al.sub.2 O.sub.3 or aluminum-chromium mixed oxides. Since in the normal sintering/infiltration process the skeleton-forming chromium grains are only slightly solubilized, such impurities cannot be released and removed by suitable purification processes or be at least diluted to harmless concentrations in the macrostructure.
The first type of defect described above is critically related to proper functioning of a vacuum switching tube. Under the action of the recurring voltage, defect-related, loosely bound contact material particles may be detached from the structure and cause a voltage puncture. The second type of defect, on the other hand, may lead to quench malfunction due to local release of gas. A simple estimate shows that for reasons of safety not more than about 1 .mu.g of gas may be released by the arc in a switching operation to avoid undue pressure peaks in the switching tube. If the quantity is contained in oxide inclusions which are reached and dissociated by the arc during quenching, one must deal with a liberated quantity of gas which impairs the switching capacity particularly at high currents with corresponding arcs of high energy density.
Tests with numerous variants of chromium-copper-based contact materials produced by singering and infiltration have shown oxide impurities with grain diameters up to 300 microns. The dissociation of these impurities would release a gas quantity of about 26 .mu.g. The possibility of reducing the average grain size during screening of the starting powders is very limited, and furthermore decreasing grain size lowers the impregnability of the Cr skeleton and therefore the first-described type of defect occurs more frequently.
It may be assumed, therefore, that when using powder-metallurgically produced chromium-copper-based contact materials in power switches, malfunctions must be expected with statistical probability.